Indicia readers (e.g., barcode scanners, OCR scanners) fall into two main classes based on their barcode-reading technology, namely (i) linear scanners (e.g., laser scanners, 1D imagers) and (ii) 2D scanners (e.g., 2D imagers, page scanners).
Laser scanners use fast moving mirrors to sweep a laser beam across a linear barcode. The bars and spaces of the barcode are recognized based on their respective reflectivity. In other words, the light areas and dark areas of the barcode reflect light back toward the scanner differently. This difference can be sensed by the scanner's photo-detector (e.g., photodiode) and converted into an electronic signal suitable for decoding.
Imaging scanners were developed to read advanced codes by adapting technology used in digital cameras. Imaging scanners take a picture of the entire barcode, and a processor running image processing algorithms recognizes and decodes the barcode. This digital approach overcomes many of the laser scanner's limitations.
Imaging scanners are more reliable than laser scanners, which use fast-moving parts. Imaging scanners can be configured to process all barcodes within a field of view and do not require separate scans for each barcode. Sophisticated decoding algorithms eliminate the need to align the imaging scanner with the barcode. Imaging scanners can also scan poor quality or damaged barcodes faster and more reliably than laser scanners. Further, the imaging scanner is more versatile and can be configured to address new codes or new modes of operation, such as document-capture. In view of these advantages, many users prefer the imaging scanner. The imaging scanner, however, lacks the extended scan range associated with laser scanners.
Extended scan ranges are important in warehouse environments, where barcoded containers may be stacked on high shelves. Operators may be limited in their access to barcodes and must scan over a range of distances. In these situations, scanning ranges can be 10 centimeters to 10 meters. This multi-order-of-magnitude range requirement places stringent demands on the imaging scanner.
The range of imaging scanners is limited by the scanner's imaging optics (e.g., lens). The quality of a barcode image is crucial for proper scans. Images that are unfocused images can render a barcode unreadable.
The range of distances over which a barcode can be decoded is known as the working-distance range. In fixed-lens systems (i.e., no moving parts), this working-distance range is the distance between the nearest focused objects and the farthest focused objects within the field of view (i.e., depth of field). The depth of field is related to the lens's f-number. A lens with a high f-number has a large depth of field. High f-number lenses, however, collect less light. Imaging scanners must collect sufficient light to prevent noisy images. These scanners, therefore, need a lens with both a low f-number and the ability to produce sharp images over a wide range of working distances. Fixed lenses, therefore, are not used for imaging scanners intended for extended range applications (e.g., warehouses).
Autofocus (i.e., AF) lenses may be used in imaging scanners that need both near and far scanning capabilities. Typically, focus is achieved in an autofocus lens by mechanically moving the lens. These mechanically-tuned autofocus lenses provide range to imaging scanners but also have some limitations.
The moving parts in mechanical autofocus lens systems may have reliability issues. The mechanical autofocus lens systems can be bulky because of the extra components required for motion (e.g., actuators, tracks, and linkages). These motion components also consume power at a rate that may limit their compatibility with battery-powered scanners. The mechanical motion of the lens or lenses can be slow and may hinder their use in applications that require fast focus (e.g., scanning in moving environments). Finally, the cost of these mechanical autofocus lens systems can be high because of the number and precision of the required mechanical parts.
Therefore, a need exists for an imaging-scanner autofocus lens system that has (i) a large focus range, (ii) a small size, (iii) low power consumption, and (iv) reduced mechanical complexity.